A significant amount of energy is used in this country to remove moisture from a wide variety of products. In many drying techniques, a fuel is combusted and the heat from the combustion is used to dry the wet materials. Nearly one quad (which is defined as 10.sup.15 B.T.U.) of energy was consumed in 1974 for drying, representing more than 1% of the energy consumption for that year. See "Analysis of the Economic Potential of Solar Thermal Energy to Provide Industrial Process Heat," COO/2829-1, 1, Intertechnology Corporation, Warrenton, VA, February, 1977.
The efficiency of a drying process depends on the specific application. However, efficiency values as low as 20-30% are common. See Perry, R. H., and Chilton, C.H., "Chemical Engineers Handbook," Fifth Edition, 20-36, McGraw-Hill Book Co., New York, NY, 1973 and Belding, J.A., and Burnett, W.M., "From Oil and Gas to Alternate Fuels: the Transition in Conversion Equipment", Energy Consumption, 17, No. 2/3, Pergamon Press, 1977.
Fuel consumption and energy considerations are important not only in drying apparatus but are important also in heat exchange apparatus generally. It is a desirable objective to provide an energy exchanging apparatus which brings about effective and efficient heat transfer from a combustion source to the walls of a heat exchanger.
Aside from the fuel consumption and energy considerations, there are also strong economic incentives to increase the efficient use of drying systems, i.e., to increase the throughput of material by more rapid drying techniques.
Drying systems based upon pulse combustors have been built in the past. These systems have exhibited both high efficiency and improved drying. See: Lockwood, R.M., "Guidelines for Design of Pulse Combustion Devices, Particularly Valveless Pulse Combustors," in Proceedings of Symposium on Pulse-Combustion Applications, 14-1 to 14-24, Atlanta, GA March 1982; Lockwood, R.M., "Sonic Energy Perforated Drum for Rotary Dryers," U.S. Pat. No. 4,334,366, June 15, 1982; Lockwood, R.M., "Pulse Combustion Fluidized Dryer," U.S. Pat. No. 4,395,830, ; Aug. 2, 1983; Severyanin, V.S., "Application of Pulsating Combustion in Industrial Installations" in Proceedings of Symposium on "Pulse-Combustion Applications," 7-1 to 7-24, Atlanta, GA, March, 1982; and "Pulse Combustion Drying," Sonodyne Industries, Portland, OR. The flow field oscillations characteristic of pulse combustors are responsible for these improvements. However, because these systems use valveless pulse combustors, noise is a severe problem.
Drying apparatus in the prior art enhance heat transfer by using flow oscillations in the drying gas. Zinn et al U.S. Pat. No. 4,529,377, for example, shows a conventional Rijke tube coupled with a drying column. The boundary condition which must exist at the junction of the two columns of FIG. 6b of the Zinn et al patent is one of atmospheric pressure. This condition is necessary to ensure the operation of the lower Rijke tube, which is the combustion source, as well as the operation of the upper Rijke tube, which is where the drying takes place. Thus with the Zinn et al device, drying takes place effectively at only a limited location in the apparatus. It would be desirable, however, if effective drying would take place in a large region in an acoustically augmented drying chamber.
The combustion which occurs in the lower Rijke tube of FIG. 6b of the Zinn et al patent varies as a function of time which is a necessary condition to establish stable oscillations in that apparatus. It would be desirable for the combustion processes providing hot gases to an acoustical drying apparatus to be steady processes.
There are configuration constraints associated with the geometry of FIG. 6b of the Zinn et al patent. The combustion Rijke tube must be below the drying Rijke tube, which may lead to actual systems of excessive height. It would be desirable to have an acoustical drying apparatus in which the configuration of a combustion source with respect to the acoustically augmented drying chamber not be severely constrained to operate efficiently.
The problem of noise reduction is recognized in U. S. Pat. No. 4,417,868 of Putnam in which noise reduction is achieved by coupling more than one pulse combustor with an inlet plenum which has a series of baffles to increase the acoustical path length. The various combustors operate out of phase, which tends to reduce noise. The main objective of the Putnam invention is to reduce the physical size of the plenum. It would be desirable to achieve noise reduction without such a complex drying apparatus. Habermehl et al., U. S. Pat. No. 4,329,141 achieves a pulsating flow by mechanically modulating the flow through the drying system with, for example, a rotating throttling valve. Inherent in this approach is a flow field which has a time varying velocity, but one which does not reverse direction. It would be desirable to have a flow field in an acoustically augmented drying chamber which would exhibit flow reversal. A reversing flow field would have significantly higher heat and mass transfer than one which does not reverse.